The use of fiber reinforced composites as replacements for metals may increase in applications requiring high strength, light weight and high operating temperatures. For example, strong, high temperature materials with low density are desirable in the manufacturing of turbine blades and rocket engines. Composites such as metal matrix composites or ceramic matrix composites may be used to extend the useful operating temperature of an article and enhance properties such as strength, stiffness, and creep resistance.
Fibers for such composites should have low reactivity, high strength and effective oxidation resistance. In addition, fiber diameters between about 13 microns and about 127 microns are desired in the manufacturing of high temperature composites.
Two high temperature fibers, SiC and Al.sub.2 O.sub.3, are currently most available. The SiC fibers, however, are fairly reactive and may not sufficiently resist oxidation. Moreover, SiC fibers of an appropriate diameter may be difficult to make due to inherent process limitations in the manufacturing of SiC fibers.
Similarly, Al.sub.2 O.sub.3 fibers may not be entirely suitable for high temperature composites. For example, Al.sub.2 O.sub.3 fibers possess low strength at elevated temperatures. Additionally, Al.sub.2 O.sub.3 fibers are single phase materials and are not alloyed for increased strength as is done with many other materials.
It is also desirable for the fiber's coefficient of thermal expansion (CTE) to be similar to the matrix's CTE to prevent a high degree of internal stress between the matrix and fibers. This stress may damage either one or both constituents. For example, SiC fibers and Al.sub.2 O.sub.3 fibers have low CTEs compared to most metals suitable for high temperature applications, such as nickel.
Accordingly, there exists a need for fibers with low reactivity, high strength and effective oxidation resistance for use in high temperature composites.